A polymer electrolyte fuel cell (sometimes abbreviated as PEFC hereafter) is highly expected to be used for a fuel cell vehicle (FCV), a stationary cogeneration system (CG-FC), etc., as it enables generation of electricity with high efficiency at a relatively low temperature.
As a catalyst used as an electrode for the polymer electrolyte fuel cell, a platinum catalyst has been used because of its high performance. However, the platinum catalyst is one of major constrictive conditions against prevalence of the polymer electrolyte fuel cell due to its limited source and high price. Especially, there is a serious problem that platinum tends to elute out from the platinum catalyst because the positive electrode (often referred to as air electrode or oxygen electrode) is surrounded by strong oxidative atmosphere due to oxygen reduction reaction when the platinum catalyst is used for the positive electrode of the polymer electrolyte fuel cell.
Therefore, extensive research and development are being done seeking the catalyst for the electrode of polymer electrolyte fuel cell which does not require expensive noble metals such as platinum (sometimes referred to as non-platinum catalyst hereafter), not only in Japan but also around the world especially in the United States.
Among such non-platinum catalysts, use of oxides and nitrides of non-platinum metals such as tantalum, zirconium, etc. has been proposed. Furthermore, carbon catalysts containing nitrogen or boron have been studied as the substantially non-metal carbon catalyst for a long time (Patent Document 1 through Patent Document 7).
For example, Patent Document 4 discloses a carbon catalyst composed of the mass of many carbon particles having a shell structure of the average diameter of 10 to 20 nm in a non-aggregated state and the effective existence ratio of nitrogen atoms in the catalyst. Patent document 5 discloses the effective existence state of nitrogen atoms in the carbon catalyst. Patent Document 6 discloses the effective existence ratio of oxygen atoms in the carbon catalyst. Various studies have been done regarding the role of nitrogen atoms in the carbon catalyst (Patent Document 7) and the reaction mechanisms have been proposed.
In addition, it has recently been reported that the carbon atoms near the nitrogen atoms at the terminus of graphite are the active sites of oxygen reduction reaction in, for example, Non-Patent Documents 1 to 3. It has also been reported that elimination of nitrogen and progress of graphite formation are promoted by the presence of metal upon carbonization of an organic polymeric material, as shown in Non-Patent Document 4.
Non-Patent Document 5 reports that a catalyst having a high catalyst activity is obtained by heat treatment of a composition of phenolic resin and iron phthalocyanine in two or more stages in combination with acid treatment after each heat treatment.
However, it is assumed that pulverization of block object is required to produce the carbon catalyst as mentioned above. Although various pulverization techniques which may enable considerable miniaturization have been proposed, this step must be incorporated in the manufacturing process, causing the cost increase in the production of the carbon catalyst. In addition, it is concerned that a surface with different catalyst property has been formed due to the appearance of a new surface by pulverization.
On the other hand, when the carbon catalyst is used as a catalyst for the air electrode of PEFC, i.e., as an oxygen reduction reaction catalyst, although the preferred reaction at the electrode is a four-electron reduction reaction of oxygen,4H++O2+4e−→2H2O    a two-electron reduction reaction2H++O2+2e−→H2O2 may proceed as a side reaction generating hydrogen peroxide in some cases. When the two-electron reduction reaction proceeds in a high proportion, not only the electricity generating property of the fuel cell decreases, but also hydrogen peroxide generated at the electrode adversely affects the fuel cell system. Therefore, the catalyst having a high efficiency in the four-electron reduction reaction is desired.
From these standpoints, presence of a plurality of surfaces on the catalyst which are different in catalyst properties will cause problems of quality control in producing a carbon catalyst having a uniform catalyst property.